You can downgrade PHP from version 7.3. to 7.2 by yum and yum-config-manager.

Following commands are what I used when downgrade PHP version 7.3 to 7.2 on Centos:

yum remove php-common php-cli mod-php
yum-config-manager --enable remi-php72
yum-config-manager --disable remi-php73
yum update
yum install php php-common php-devel php-bcmath php-gd php-pdo php-pear php-mysqlnd php-mbstring php-xml php-tidy php-soap php-pecl-apcu php-pecl-ssh2 php-mcrypt php-pecl-memcache php-intl php-zip redis ImageMagick-devel